Telomeres are repetitive DNA sequences accompanied by proteins that cap and protect the end of each chromosome from continuous degradation in each cell cycle, thereby securing and protecting chromosomal integrity. Telomere shortening may also lead to cancer by contributing to genomic instability (Raynaud et al., Crit. Rev Oncol Hematol 66:99-117, 2008), and has been associated with aging and cellular senescence (Yang, Cytogenet Genome Res 122:211-218, 2008). It is well established that telomeres get gradually shorter during the course of normal aging. It has been reported that up to 200 base pairs of telomere DNA are lost with each round of DNA replication. For example, in new-born humans, peripheral blood lymphocytes have approximately 10 kb of telomere DNA at both ends of each chromosome, which gradually shorten to approximately 6 kb by the age of 70. It is also known that environmental factors and life-style factors can accelerate telomere shortening. It is believed that such telomere shorting is associated with age-related cellular decline. It is also believed that telomere shortening limits the number of cell divisions, which ultimately results in limiting human life span. It is also known that humans are born with differing lengths of telomeres. For example, some humans start with approximately 8 kb of telomeres, while others start with approximately 12 kb of telomeres. Accordingly, humans with shorter telomeres may be more susceptible to developing certain age-related pathological conditions at an earlier age than those with longer telomeres. Such pathological conditions include, for example, immunological deficiencies, chronic ulcers, atherosclerosis, age-related blindness due to a proliferative decline of retinal pigmented epithelial cells, and cancer.
Moreover, there are various diseases and disorders that are also associated with telomere shortening (Armanios and Blackburn, Nat Rev Genet. 2012 October; 13(10):693-704). Examples of genetic diseases that can cause telomere shortening include dyskeratosis congenita, Hoyeraal-Hreiderasson syndrome, Revesz syndrome, and Coats plus syndrome. Additionally, it was recently shown that a significant fraction of idiopathic pulmonary fibrosis (IPF) is caused by telomere shortening. Similarly, some liver cirrhosis and pancreatic fibrosis may be caused by telomere shortening. Considering the prevalence of such pathological conditions, it appears that diseases caused by telomere shortening are more common than previously thought.
Another example of a disease associated with telomere shortening is Fanconi anemia. Fanconi anemia is a rare autosomal recessive disease. Fanconi anemia is an inherited bone marrow failure syndrome that is characterized by progressive pancytopenia and cancer susceptibility (Bogliolo et al., Mutagenesis. 2002 November; 17(6):529-38). It has been reported that Fanconi anemia patients show accelerated telomere shortening (Leteurte et al., Br. J. Haematol., 1999; Ball et al., Blood, 1998; Hanson et al., Cytogenet. Cell Genet. 2001; and Callen, et al., Hum Mol Genet. 2002 Feb. 15; 11(4):439-44).
One potential method of treating these various telomere shortening-associated diseases and disorders is to use telomerase to lengthen the shortened telomeres. Telomerase has been identified as the major enzyme known to be involved in telomere elongation maintenance. While telomerase is active in embryonic stem cells, telomerase is usually not expressed in non-embryonic (i.e., adult cells), such as somatic cells. Thus the reactivation of telomerase or forced expression of telomerase in adult cells may be used to increase telomere length. However, one potential problem with the use of telomerase is that the continuous expression of telomerase is often associated with tumorigenesis and cancerous transformation. Accordingly, expression of telomerase is not an ideal way to increase telomere length in patients suffering from diseases or conditions associated with telomere shortening.
Another potential method to lengthen telomeres is to use a recently discovered component of a Chinese herb (TA-65) that can potentially increase telomere length (Harley et al., Rejuvenation Research 14:45-56, 2011). However, it has not been well established that this herb can effectively lengthen telomeres. Moreover, use of this herb would require long-term continuous administration of drugs to treat patients in need of telomere lengthening.
Additionally, it has recently been shown that Zscan4 (Zinc finger and scan domain-containing protein 4) is required for the maintenance of genome stability and normal karyotype in mouse embryonic stem cells and is expressed in mouse embryos and embryonic stem cells (Falco et al., Dev Biol 307:539-550, 2007; Zalzman et al., Nature 464:858-863, 2010; PCT Publication Nos. WO 2008/118957, WO 2011/02880, WO 2012/103235, WO 2012/129342, WO 2012/158561, and WO 2012158564; and U.S. Patent Application Publication Nos. US 2010/0105043, US 2012/0129161, and US 2012/0156305). It has also been shown that Zscan4 expression in mouse embryonic stem cells is associated with telomere elongation (Zalzman et al., Nature 464:858-863, 2010; PCT Publication Nos. WO 2011/02880, WO 2012/129342, and WO 2012158564; and U.S. Patent Application Publication No. US 2012/0156305). While, it has been shown that the human genome also contains a Zscan4 gene, none of Falco et al., Dev Biol 307:539-550, 2007; Zalzman et al., Nature 464:858-863, 2010; PCT Publication Nos. WO 2008/118957, WO 2011/02880, WO 2012/103235, WO 2012/129342, WO 2012/158561, and WO 2012158564; or U.S. Patent Application Publication Nos. US 2010/0105043, US 2012/0129161, and US 2012/0156305 provide experimental support demonstrating that Zscan4 expression leads to same effects in human cells as it does in mouse embryonic cells. It is particularly unclear whether human Zscan4 would have the same function as mouse Zscan4, as the mouse genome contains six Zscan4 genes and three Zscan4 pseudogenes while the human genome only contains one Zscan4 gene (PCT Publication No. WO 2008/118957). Moreover, it is unknown whether Zscan4 expression in human non-embryonic cells, such as the somatic cells involved in diseases and conditions associated with telomere shortening, would have the same effect as shown for mouse embryonic stem cells.